In certain applications, it is important to keep track of the velocity of a rotating drum. For example, an engineering print or other document may be mounted to a drum of an optical document digitizer. In a prior art device, the drum is carried on a shaft which is rotatably mounted by bearings to a rigid frame. As the drum rotates, a document on the drum is scanned by an optical scanner including an array of photodetectors. The output of each photodetector is repeatedly integrated over time to produce digital data corresponding to the portion of the document which is scanned by the photodetector during the integration time. To insure that a uniform area of the document is scanned during each integration time interval, the velocity of the drum is monitored and the duration of the integration time interval is adjusted with variations in drum velocity. That is, the integration time is increased with decreasing drum velocities and decreased with increasing drum velocities. An optical encoder is rigidly mounted to the frame of such device in a position to sense rotations of the drum shaft and thereby monitor the velocity of the drum.
However, during normal use, the frame of these devices is occasionally bumped or jarred. This jarring can result in rotational displacement of the drum supporting frame relative to the portion of the frame which supports the optical encoder. When this occurs, the optical encoder incorrectly attributes this displacement to a change in the velocity of the drum and produces an erroneous velocity reading. As a result, integration time is incorrectly adjusted. Consequently, when the digitized data is used to reproduce copies of the print, the copies contain distortions.
Therefore, need exists for an improved mounting system for a rotatable drum and an optical encoder which is directed toward overcoming these and other disadvantages of prior art devices.